Non-volatile memories include memories which retain data even when the power supply is turned off. Typical memories in this category are various read only memories (ROMs), such as mask programmable ROMs, erasable programmable ROMs (EPROMs), and electrically erasable read only memories (EEPROMs). These memories typically have memory cells which are either in a high conductivity or low conductivity state. In the case of EPROMs and EEPROMs, the memory cells have either a relatively high threshold voltage or a relatively low threshold voltage which is used to obtain the high and low conductivity states. Another typical characteristic is that the memory cells have a single output which is connected to a bit line. Consequently, a number of sense amplifiers for ROMs were single-ended. More recently, however, it has been more common to use reference cells (also commonly called dummy cells) so that differential amplifying techniques could be used.
Differential techniques have long been used in random access memories (RAMs) such as dynamic RAMs (DRAMs) and static RAMs (SRAMs). These techniques have been primarily voltage oriented approaches because of the charge storing aspect of DRAMs and the inherent voltage differential of SRAMs. Modification of these techniques for use in non-volatile memories has resulted in improved sensing. One example of this is shown in FIG. 2 of "A Programmable 80ns 1Mb CMOS EPROM," Saito et al., pages 176-177, DIGEST OF TECHNICAL PAPERS, 1985 IEEE International Solid-State Circuits Conference. This approach uses the inherent current carrying aspect of non-volatile cells to generate a voltage differential which is then amplified by a voltage differential amplifier. This technique, however, is still primarily a differential voltage amplifier technique. One problem is that there is a significant performance dependence on changes in process parameters. The bias point established on the differential voltage amplifier is dependent upon a ratio of conductivities of a P channel transistor and an N channel floating transistor. Although transistors of the same type track very well over process variations, transistors of different types do not. There is some tracking between regular N and P channel transistors and between regular N channel and N channel floating gate transistors. It is thus desirable that performance should be ratioless rather than depend upon the ratio of transistors of differing types.